This invention generally relates to a method of specialized induction heat treating and restraint quenching of structural members which generally are in the shape of a channel but can be any member having a web and at least one flange. The object of this specialized heat treatment would be to produce certain physical properties which are required in the design of truck side rails, as a means of increasing strength of a given side rail section such that the payload capability of a truck is increased with no additional weight penalty in the side rail effecting a more efficient vehicle. Truck side rails are long structural channel members generally 8 to 40 feet in length, 0.250" to 0.375" thick. Due to the lean alloy composition of the low carbon steel specified in this invention, a rapid effective quench is necessary to obtain the required minimum physical properties and microstructure. As the truck side rail is a long, thin section member, generally cold formed into its channel shape from a flat strip, considerable residual stresses are present due to the forming operation. During the heating and rapid quenching of these thin section members, considerable distortion occurs due to the interaction of the residual stresses and the severe thermal action of the quench; distortion is evident in the member as bow, twist, and camber, as well as growth experienced along the length of the side rail due to the material grain size enlargement as a result of the heat treat operation.
This invention further relates to a method of induction heat treating and restraint quenching structural members such that the total system represents a more economical investment than do existing conventional methods of restraint quenching, in particular, restraint quenching truck side rails. These other techniques for restraint quenching truck side rails are taught in the Kuchera U.S. Pat. Nos. 3,252,695 and 3,294,597 in which are employed large movable die quench sections constructed with a vast network of quench fluid passages to simultaneously allow quench fluid passage and provide restraint, such that the overall effect is akin to a large forming press complete with dies, representing a sizeable investment. The method of restraint taught in conventional methods must be elaborate as the entire member is quenched at the same time, setting up significant thermal gradients along the member length, greatly increasing the propensity to distort. This invention does not require the large quench press and its attendant special die sections as relatively short sections of the rail are heated in stages to austenitizing temperature and then immediately restraint quenched on a continuing basis as the member travels longitudinally through the system, necessitating only a relatively short restraint quench system which, in this invention, employs restraint rolls similar to those found in roll forming operations.
It is known generally to produce induction heated, quenched and tempered, high strength steels for tubular products but not necessarily for structural members, more specifically channel members. One method of making these is described by Tom Allan in a paper entitled "Production of High Strength Quenched and Tempered ERW Tubular Products Utilizing Induction Heating Equipment," 13th Mechanical Working and Steel Processing Conference, Jan. 20, 1971, Pittsburgh, Pennsylvania.
Generally others have taught various techniques for rapid heat treating of steel and quenching. Examples of this are R. A. Grange, "The Rapid Heat Treatment of Steel", Metallurgical Transactions, Vol. 2, January 1971, p. 65-78. More generally, of course, the induction and flame hardening of metal is known in the Metals Handbook, Vol. 2, 8th Edition, "Induction and Flame Hardening", pages 167-202. The particular type of hardening taught in this instance is known as "through hardening." Other developments of bar quenching fixtures are taught in U.S. Pat. No. 3,738,629. Various inductor coil designs are taught in patents, such as U.S. Pat. No. 3,699,302 and page 171 of the Metals Handbook, Vol. 2.